Pressure-induced structural and electronic transitions with negative compression in Co2(OH)3Cl kagome lattice nanoparticles

Geometrically frustrated materials with kagome lattices have garnered significant research interest due to their unique topological properties and exotic magnetic behavior. Among these kagome materials, $\mathrm{C}{\mathrm{o}}_{2}{(\mathrm{OH})}_{3}\mathrm{Cl}$ stands out for its pronounced crystal field effect, which invokes the highly pursued exploration of the interplay among the crystal structure, coordination environment, and electronic behavior. In this paper, pure-phase $\mathrm{C}{\mathrm{o}}_{2}{(\mathrm{OH})}_{3}\mathrm{Cl}$ nanoparticles were successfully synthesized via a hydrothermal reaction. The morphology, crystal structure, and bonding characteristics were comprehensively characterized at ambient conditions. Low-temperature magnetic measurements revealed a ferromagnetic transition at $\ensuremath{\sim}10$ K. The structural evolution of $\mathrm{C}{\mathrm{o}}_{2}{(\mathrm{OH})}_{3}\mathrm{Cl}$ upon compression was further investigated using in situ high-pressure synchrotron x-ray diffraction (XRD). At 10.8 GPa, an anomalous negative compression occurs along the $c$ axis. A structural transformation from the initial rhombohedral to the monoclinic phase has been uncovered at 13.5 GPa, accompanied by a significant increase in bulk modulus. High-pressure Raman scattering and ultraviolet-visible absorption measurements indicated that the phase transition was driven by an increase in crystal field splitting energy ($\mathrm{\ensuremath{\Delta}}$), which enhanced the Jahn-Teller effect, altered the electronic configuration ($p\ensuremath{-}d$ hybridizations and $d\text{\ensuremath{-}}d$ transition) of $\mathrm{C}{\mathrm{o}}^{2+}$ ions, and potentially induced a transition from high- to low-spin states. In this paper, we provide insights into the mechanisms underlying structural transitions and electronic behavior in geometrically frustrated materials with kagome lattices under external pressures.